1. Field of the Invention
The present invention relates to semiconductor substrate handling systems and in particular relates to semiconductor substrate pickup devices employing gas flow to lift a wafer in a substantially non-contacting manner.
2. Description of the Related Art
Integrated circuits are typically comprised of many semiconductor devices, such as transistors and diodes, which are formed on a thin slice of semiconductor material, known as a wafer. Some of the processes used in the manufacturing of semiconductor devices in the wafer include an epitaxial process or a doping process that involves positioning the wafer in high temperature chambers where the wafer is exposed to high temperature gases which result in doped layers or regions being selectively formed in the wafer. When forming such integrated circuits, it is often necessary to remove the wafer from one high temperature chamber having a first doping or epitaxial species and reposition the hot wafer having a temperature as high as 1200 degrees Celsius to another high temperature chamber having a different doping or epitaxial species. However, since the wafer is extremely brittle and vulnerable to particulate contamination, great care must be taken so as to avoid physically damaging the wafer while it is being transported, especially when the wafer is in a heated state.
To avoid damaging the wafer during the transport process, various well known wafer pickup devices have been developed. The particular application or environment from which the wafer is lifted often determines the most effective type of pickup device. One class of pickup devices, known as Bernoulli wands, are especially well suited for transporting very hot wafers. The advantage provided by the Bernoulli wand is that the hot wafer generally does not contact the pickup wand, except perhaps at one or more small locators positioned on the underside of the wand. Such a Bernoulli wand is shown in U.S. Pat. No. 5,080,549 to Goodwin, et al.
In particular, when positioned above the wafer, the Bernoulli wand utilizes jets of gas to create a gas flow pattern above the wafer that causes the pressure immediately above the wafer to be less than the pressure immediately below the wafer. Consequently, the pressure imbalance causes the wafer to experience an upward "lift" force. Moreover, as the wafer is drawn upward toward the wand, the same jets that produce the lift force produce an increasingly larger repulsive force that prevents the wafer from substantially contacting the Bernoulli wand. As a result, it is possible to suspend the wafer below the wand in a substantially non-contacting manner. However, Bernoulli wands known in the art do not always operate in the most advantageous manner.
In particular, although heat conduction from the hot wafer to the Bernoulli wand is substantially minimized, other modes of heat loss from the wafer are likely. Specifically, the wafer emits thermal radiation or radiant heat, at a rate that is proportional to the fourth power of the temperature of the wafer. Furthermore, the moving gas at the upper surface of the wafer caused by the jets of gas emanating from the Bernoulli wand is likely to cause the wafer to experience significant convective heat loss. Moreover, since the spacing between the wafer and the wand is small, conduction through the gas is a third significant heat loss mechanism. Consequently, it is likely that the internal energy of the wafer will drop significantly while the wafer is moved by the wand between high temperature chambers, thus causing the temperature of the wafer to decrease significantly during the movement process.
The possible reduction in temperature of the wafer resulting from the movement of the substrate may be desirable when high temperature processing is complete but in many circumstances is undesirable. In particular, if significant cooling occurs during the movement process, additional time is required in the manufacturing process so as to allow the wafer to achieve a preferred target processing temperature when manipulated between high temperature chambers. Of even greater concern, however, is the possibility that the cooled wafer will deform and experience thermal shock when abruptly placed in a hot reactor or onto a hot body, thereby possibly damaging the wafer. Furthermore, when a cooled wafer is placed on a hot body such as a susceptor, it is possible for the susceptor to experience deleterious thermal shock, which can damage the susceptor.
From the foregoing, it will be appreciated that there is a need for a semiconductor wafer pickup device that enables a high temperature wafer to be transported within a semiconductor processing system in a manner to reduce the likelihood of damaging the wafer and sensitive components of the semiconductor processing system. To this end, there is a need for a pickup device that regulates the temperature of the wafer during the manipulation process.